A motor controller or drive is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A drive can include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults, for example. Many industrial applications are dependent upon motors (or machines), which range from the very small to the size of a railroad locomotive. The motor controllers or drives can be built into the driven equipment, installed separately, installed in an enclosure along with other machine control equipment.
An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet, servo, series, separately excited, and alternating current. The motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form of analog or digital input signals. Recent developments in drive electronics have allowed efficient and convenient acceleration, speed, and position control of these motors, where this has not traditionally been the case. The newest advancements allow for torque generation down to zero speed. This allows a poly-phase AC induction motor to compete in areas where DC motors have long dominated, and present an advantage in robustness of design, cost, and reduced maintenance.
Phase vector drives (or simply vector drives) are an improvement over variable frequency drives (VFD) in that they separate the calculations of magnetizing current and torque generating current. These quantities are represented by phase vectors, and are combined to produce the driving phase vector which in turn is decomposed into the driving components of the output stage. Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position and phase currents. Rotor position can be obtained through an encoder, but is often sensed by the reverse EMF generated on the motor leads. In some configurations, a vector drive may be able to generate full rated motor torque at zero speed.
Another aspect of drives includes brushed DC Motor Speed or Torque Controls. These controls are applicable to brushed DC motors with either a wound or permanent magnet stator. A valuable characteristic of these motors is that they are easily controlled in torque, the torque being fairly directly proportional to the driving current. Speed control is derived by modulating the motor torque. SCR controls for DC motors derive power from AC power, and send rectified voltage to the motor. These controls are very common in industry, running from line voltages, with motors rated at 90V for 120V line, and 180V for a 240V line. These are available in reversing and non-reversing models and are robust, with a minimum of electronic components. The waveform sent to the motor can have strong harmonic components due to the switching at line frequency. This results in current and torque ripple, and an audible hum.
Pulse width modulated (PWM) controls use pulse width modulation to regulate the current sent to the motor. Unlike SCR controls which switch at line frequency, PWM controls produce smoother current at higher switching frequencies, typically between 1 and 20 kHz. At 20 kHz, the switching frequency is inaudible to humans, thereby eliminating the hum which switching at lower frequency produces. However, some motor controllers for radio controlled models make use of the motor to produce audible sound, most commonly simple beeps.
In the past, regardless of the type of motor controller or drive employed, power electronics and control electronics for the drive were integrated into a common control package or platform. Thus, traditional industrial manufacturing drives feature fixed, immovable hardware. The drawback to these traditional drives is that technology is constantly improving. Most manufacturing companies are faced with the option of replacing an entire drive, or settling for out-dated technology that does not accurately meet their needs. The current competitive business world makes both of these options unappealing.